Answer : The correct answer is option D. N₂(g) at 25.0°C and 1 atm pressure
Explanation :
Standard heat of formation defined as the change in heat when one mole of a compound is formed from its elements in standard state.
Therefore all the substances which are compounds will have some value of heat of formation.
On the other hand, a substance will have zero heat of formation when it is present in elemental form and in its standard state at STP
If we take a look at the given options, we can see that Pb and N₂ exist in elemental forms.
But we have Pb at 1000° C. Melting point of Pb is around 620° C. Above this temperature, Pb will be present in liquid state which is not its standard state. (standard state of Pb is solid as it is a metal)
Therefore the only substance that is present in elemental form and in its standard state is N₂.
The correct option is N₂ (g) at 25°C and 1 atm
